Nitride semiconductors such as GaN, AlN, and InN, or mixed crystals of these materials have wide band gaps, and are used as high-output electronic devices, short-wavelength light-emitting devices, and the like. To be used as high-output devices, technologies relating to field-effect transistors (FET), especially, high electron mobility transistors (HEMT) have been developed. A HEMT that uses such a nitride semiconductor is used for a high-output, high-efficiency amplifier, a high-power switching device or the like because it can operate with great current, high voltage, and low on-resistance.
Among such nitride semiconductors, a HEMT having an InAlN/GaN structure that uses InAlN as an electron supply layer has great spontaneous polarization of InAlN, and can generate greater 2 DEG than a HEMT having an AlGaN/GaN structure. Therefore, the HEMT having an InAlN/GaN structure can have the on-resistance lower than the HEMT having an AlGaN/GaN structure, with which characteristics of the semiconductor device can be improved.